The use of threaded tubular connections for joining flow conduits in an end-to-end relationship to form a continuous flow path for transporting fluid under pressure is well known. Oilfield tubular goods all use threaded connections for connecting adjacent sections of conduit or pipe. Examples of such threaded end connections designed for use on oilfield tubular goods are disclosed in U.S. Pat. Nos. 2,239,942; 2,992,019; 3,359,013; RE 30,647; and RE 34,467, all of which are assigned to the assignee of the present invention.
Current technology exists that combines a conical metal-to-metal seal with a wedge thread as the torque stop. The difficulty in working with this design concept is that the linear variance of the wedge torque stop, due to the wedge thread geometry and process capabilities, requires a very shallow angle for the metal-to-metal seal. In order to generate enough radial interference in the metal-to-metal seal to effect an efficient sealing mechanism, a great deal of rotation is required from the time the shallow angle seals of the pin and box members make initial contact to the point of final make up. The longer metal-to-metal seals are in contact during rotation, the higher the tendency for galling. If interference is decreased to lessen the amount of rotational contact, sufficient contact forces may not exist to effect a reliable seal at final make up. This results in a critical balancing act that affords little leeway along the sealing ability versus galling resistance continuum.
As shown in FIG. 1, a prior art connection 10 includes a pin member 11 and a box member 12. Box member 12 has a tapered, internal, generally dovetail-shaped thread structure 14 formed thereon and adapted for engaging complementary tapered, external, generally dovetail-shaped thread structure 15 formed on pin member 11 to mechanically secure the box and pin members in a releasable manner.
Internal thread 14 of box member 12 has stab flanks 18, load flanks 16, roots 20, and crests 24. The thread increases in width progressively at a uniform rate in one direction substantially the entire helical length of thread 14. External thread 15 of pin member 11 has stab flanks 19, load flanks 17, roots 21, and crests 25. The thread increases in width progressively at a uniform rate in the other direction substantially the entire helical length of thread 15. The oppositely increasing thread widths and the taper of threads 14 and 15, cause the complementary roots and crests of the respective threads to move into engagement during make-up of the connection. Root and crest engagement is followed by the moving of complementary stab and load flanks into engagement upon make-up of the connection. The moving of complementary flanks, roots and crests into engagement forms sealing surfaces that resist the flow of fluids between the threads
The pin member 11 or the box member 12 defines the longitudinal axis 13 of the made-up connection 10. The roots and crests of the box and pin members are flat and parallel to the longitudinal axis of the connection and have sufficient width to prevent any permanent deformation of the threads when the connection is made up.
FIG. 2 shows a profile of the wedge thread on pin member 11. The dimension AA represents the location of the thread start and dimension BB represents the thread depth. The stab flank and load flank leads are denoted CC and DD respectively.